This invention relates generally to the real-time detection of chemical species, and more particularly, relates to a method and apparatus for the detection, identification and quantification of chemical species using acoustic waves.
Real-time detection of chemical species is necessary to protect the environment, to minimize worker exposure to toxic chemicals and to efficiently control industrial processes. The detection, identification and quantification of dilute concentrations of chemical species in ambient environments or industrial processes generally require sophisticated analytical equipment such as gas chromatography and mass spectrometry. The cost and bulkiness of these devices and the long times required to perform a chemical analysis make these devices inappropriate for real-time detection of chemical species.
Chemical sensors provide an inexpensive, sensitive and small apparatus for detecting chemical species One type of chemical sensor is based on the use of acoustic wave (AW) devices, particularly surface acoustic wave (SAW) devices, but also including acoustic plate mode (APM) devices and Lamb wave devices, use two interdigitated electrodes on a piezoelectric substrate such as quartz to launch and detect an acoustic wave which can interact with the environment or a coating applied to the device substrate while travelling between the two electrodes. As an example, a method and apparatus for using a SAW device to detect a chemical species in a fluid is disclosed in U.S. Pat. No. 4,312,228, entitled "Methods of Detection with Surface Acoustic Wave and Apparati Therefor" to Wohltjen and is further described in H. Wohltjen, published in SENSORS AND ACTUATORS, Vol. 5, pp. 307-325 (1984). Wohltjen teaches that chemical species can be detected with SAW devices, the surfaces of which are coated with materials which selectively interact with a different chemical species. The interaction between the chemical species and the surface coating is detected by monitoring changes in the frequency, the phase or the attenuation of the surface acoustic wave. The magnitude of the change of one of these properties of the surface acoustic wave indicates the concentration of the species interacting with the surface material.
M. S. Nieuwenhuizen and A. Venema in an article entitled "Surface Acoustic Wave Chemical Sensors", SENSORS AND MATERIALS, Vol. 1, pp. 261-300 (1989) further describe selective coatings for SAW devices and how these selective coatings are used to prepare coated SAW devices to detect a particular chemical species. But there are only a few examples where a coating material preferentially interacts with one species to the exclusion of others. Moreover, these coating materials often irreversibly bind the chemical species of interest because of the strong chemical interactions required to achieve this level of selectivity; thereby reducing sensor lifetime and decreasing the maximum detectable concentration. Because these coatings only detect one species, another shortcoming of this type of coated SAW device is that for each chemical species to be detected, a separate coated SAW device is required, which for some practical applications where it is desirable to detect a large number of chemical species, an unreasonably large number of coated SAW devices is necessary.
To overcome these shortcomings, an alternative approach is the use of an array of SAW devices with coating materials which, while sorbing a variety of chemical species, exhibit some selectivity in the amount of sorption. Thus, the relative magnitudes of the responses of different coated SAW devices will depend on the chemical species providing the responses. Using pattern recognition techniques, the responses from such an array of coated SAW devices can be analyzed to determine the presence and concentration of the chemical species providing the responses. Such use of pattern recognition schemes for data analysis from an array of chemical sensors has been described by Carey et al., ANALYTICAL CHEMISTRY, Vol. 58, pp. 149-153 (1986) and Rose-Pehrsson et al. in ANALYTICAL CHEMISTRY, Vol. 60, pp. 2801-2811 (1988).
Practical application of chemical sensors arrays and pattern recognition schemes has been hindered by the need for multiple sensors, by variability in preparation of the coating materials and the difficulty of obtaining discriminating coating materials for certain chemical species. Typically, four to twelve different coated sensors in an array are used and the complexity and expense increase with the number of sensors. The variability in the coating materials from one sensor array to another requires elaborate calibration of each sensor array corresponding to the chemical species of interest for accurate detection and quantification. Finally, because the magnitude of responses to various chemicals is dependent upon the chemical interactions between the species and the coating materials, and because many chemical species have similar chemical properties to other chemical species which may be present, it is difficult to prepare a set of coating materials which can discriminate between similar species.
Another technique for identifying chemical species which provides a SAW device response is disclosed in U.S. Pat. No. 4,895,017, entitled "Apparatus and Method for Early Detection and Identification of Dilute Chemical Vapors" to Pyke et al. In this technique, the transient responses of one or more coated SAW devices are monitored to determine the time constants for sorption into the coating materials. Time constants vary significantly for different chemical species, therefore the time constants empirically identify the species providing the responses and the magnitudes of the responses determine the concentration of the sorbing species. This technique has the disadvantage of requiring a sophisticated analytical procedure such as, for example, a Kalman filter technique to efficiently analyze the transient response and determine the time constant. Note that the time constant for sorption is significantly larger than the sampling period for the SAW sensor responses, therefore an apparatus which permits rapid sampling is required. But coating materials which provide rapid sorption properties are ineffective with this technique because the time constant for sorption is too small to be quantitatively identified. The discriminating ability increases if the time constant for sorption of different chemical species varies dramatically. However, this limits the speed of the sensor since an analysis of the transients for the long time constant species will require monitoring the responses for long times and, in order to remove the sorbed species after detection to regenerate the sensor, even longer times will be required to fully desorb the chemical species from the coating material. An additional complication is the fact that the time constant for sorption can depend on the concentration of the sorbing species as well as the presence or absence of other chemical species.